Cyclin-dependent kinases (CDKs) are key regulators of cellular proliferation of eukaryotes. The CDKs are the ultimate targets of growth stimulatory and inhibitory signals. Deregulation of CDK activities has been found in a variety of human cancers. Thus, to learn how CDKs execute their functions is not only necessary for elucidating the processes that control cellular proliferation, but also provides insights into mechanisms of tumorigenesis. The long-term objective of our research is to understand the detailed molecular mechanisms by which cell division cycle is regulated in human cells and to understand how the perturbation of normal cell cycle control results in tumorigenesis. The regulation of CDKs has been extensively studied, nevertheless, relatively little is known about how these CDKs regulate specific cell cycle events, largely because physiological substrates of these kinases have not been adequately characterized. To understand how cyclin E-Cdk2, a key CDK required for the G1-to-S phase transition, executes its function in vivo, we developed a strategy to isolate potential cyclin E-Cdk2 substrates and identified NPAT as one of the targets of this kinase complex. We and others have shown that NPAT activates histone gene transcription and promotes S phase entry. These results suggest that NPAT plays an important role in the G1-to-S phase transition. Here we propose to investigate the molecular mechanisms of both NPAT function and regulation. The specific aims of this grant are to: (1) investigate the mechanism by which NPAT coordinates transcriptional histone gene transcription; (2) determine the roles of NPAT in histone gene transcription and G1/S phase transition; (3) study the regulation of NPAT function by cyclin E-Cdk2 kinase. It is not unreasonable to expect that this research may lead to identification of novel targets for cancer diagnosis or therapy.